Does Agonizing Blast add damage per Eldritch Blast casting, or per beam?

@DackeStaffanDoes Agonizing Blast add damage per Eldritch Blast casting, or per beam? E.g. 5th level lock deals 2d10+2*Cha, or 2d10+Cha? I would rule that you add your Charisma modifier whenever a beam hits. But I have my eye on this feature.